The present invention relates to transducers for providing an electrical signal indication in real time of the instantaneous condition of a fluid in an operating system. In particular, the invention relates to fluid condition monitoring in fluids employed for engine lubrication and for hydraulic actuation in power transmission devices such as automatic transmissions for motor vehicles.
It is known to utilize impedance spectroscopy or impedance measurements at different frequencies associated with the bulk fluid impedance and the surface impedance of the electrodes for determining the changed condition of an operating fluid over time and service as described in U.S. Pat. No. 6,278,281 in the name of R. A. Bauer, et al and assigned to the assignee of the present invention. The aforesaid fluid condition monitor utilizes a pair of electrodes disposed in a generally planar array with interdigitated elements thereof or spaced parallel plates and disposed for immersion in the fluid to be monitored. The disadvantage of the probe arrangement of the aforesaid ""281 patent to Bauer, et al is that of the difficulty of supporting and shielding such an arrangement from mechanical interference such as that due to vibration or fluid movement such as splashing or pressure perturbations where the probe is intended to be immersed in motor vehicle transmission fluid or engine lubricant at elevated temperatures.
Furthermore the arrangement of the electrode elements in the aforesaid probe and particularly where the electrodes are formed of thin material deposited upon a substrate has proven to be difficult to arrange to provide a required surface area and spacing, not sufficiently robust and relatively high cost in manufacture.
Furthermore, problems have been encountered in providing a sufficiently high signal to noise ratio for probes of the aforesaid type in impedance measurement type fluid monitoring. Therefore, it has been desired to provide an improved probe for a transducer employing the aforesaid impedance spectroscopy techniques for monitoring the condition of a fluid in real time with respect to fluid contamination and constituency changes and to provide such an improved probe that is reliable in harsh service environments and easy to manufacture and relatively low cost at the high volumes required for automotive applications.
The present invention provides a probe assembly intended for use in a transducer utilizing impedance spectroscopy by measuring impedance at different frequencies associated with bulk fluid impedance and surface electrode impedance for monitoring in real time the condition of a fluid such as hydraulic fluid, including automatic transmission fluid and mineral based engine lubricating oil and employs a pair of concentric tubular electrodes for immersion in the fluid to be monitored.
The electrodes of the invention probe assembly may be electrically connected from a correspondingly disposed end thereof attached to a header. If desired, each tubular electrode may have an integral connector terminal formed extending axially therefrom.
The inner tubular electrode has a minimum surface area disposed at a predetermined distance from the inner periphery of the outer tubular electrode. Where the electrode assembly is immersed in fluid contained in a vessel capable of acting as a Faraday shield, in one embodiment, the probe assembly does not require a shield of its own; whereas, if the electrodes are immersed in fluid in a vessel or casing which does not act as a Faraday shield, in another embodiment, the probe employs its own outer tubular Faraday shield. The present invention provides a probe assembly having a pair of concentrically spaced tubular electrodes suitably configured for impedance spectroscopy measurement of fluid condition in real time where the electrode material provides a robust configuration and has a minimum bulk and is easily configured for installation through an aperture formed in the wall of the fluid containing vessel, such as an engine crankcase or transmission casing. Furthermore the present invention provides electrodes arranged for improved signal to noise ratios thereby minimizing the amount of signal conditioning required for remote signal processing. Faraday shield housing can reduce fluid turbulence, maintain better isothermo condition in addition to providing zero E field enclosure to prevent sensing current leakage, and further eliminating EMI.